In addition to airbag systems for protecting the passengers of an automobile, the development has also recently been desired of airbag systems for the protection of individuals (pedestrians and the like) subjected to impact by an automobile.
Since the gas generator in a pedestrian-protective airbag system is installed outside the cabin (the interior of the automobile in which the passengers reside), it is readily subject to influences from the outside environment and to influences such as heat from the automobile engine or the like, making the implementation of countermeasures to these influences a necessity.
The claims in WO-A No. 96/27574 describe a gas generating composition comprising the combination of nitroguanidine and phase-stabilized ammonium nitrate; this gas generating composition is thermally stable and has a melting point of not less than 100° C. In addition, in the examples, the ammonium nitrate exhibits phase transitions at 120° C. and 130° C., and volumetric changes therefore occur when left at these high temperatures that make it impossible for the gas generating composition to retain its shape.
The claims in WO-A No. 98/04507 describe a thermally stable non-azide gas generating agent comprising a combination of nitroguanidine, tetrazoles, and triazoles as fuel with phase-stabilized ammonium nitrate or ammonium perchlorate as oxidizing agent. When phase-stabilized ammonium nitrate is added, however, the ammonium nitrate undergoes phase transitions at 120° C. and 130° C., and significant volumetric changes therefore occur when left at these high temperatures that make it impossible for the gas generating composition to retain its shape. In addition, tetrazoles and triazoles react even at 100° C. with phase-stabilized ammonium nitrate and decompose, generating ammonia gas. Ammonium perchlorate is unsuitable for use in airbag systems because it produces large amounts of gases such as hydrogen chloride and chlorine during combustion.
The claims of WO-A No. 03/011798 describe a highly thermally stable and strongly cohesive gas generating agent that comprises cationic polymer and anionic polymer and also binder. However, at the level of specifics, the operating temperature range for the gas generating agent is limited to −40° C. to 120° C., and only a 107° C.×400 hr high-temperature storage test and a heat shock test at the level of −40° C. to 100° C.×200 cycles are conducted in the examples.
The heat resistance of the gas generating agents according to the inventions in the aforementioned WO-A No. 96/27574, WO-A No. 98/04507, and WO-A No. 03/011798 is inadequate when the gas generator is installed in a location readily susceptible to automobile-induced thermal effects, for example, around the engine compartment.